Diverse communities broaden ecosystem function




  • Species occur together
    • not all perfectly adapted
    • networks of trophic interactions


  • Ecosystems work better as a group
    • No single species performs all roles
    • larger threshold of environmental conditions
    • fosters ecosystem resilience
      • ball & cup model

Diverse communities broaden ecosystem function




  • Species occur together
    • not all perfectly adapted
    • networks of trophic interactions


  • Ecosystems work better as a group
    • No single species performs all roles
    • larger threshold of environmental conditions
    • fosters ecosystem resilience
      • ball & cup model

Not all species are created equal

Reintroduction of wolves in Yellowstone: Top down control

Hierarchical levels in an community: Food webs

Energy flow limits length of food chain (10% rule)

Building ecosystems: Succession

Disturbance & succession

Disturbances are the major cause of fluctuation in succession


  • Discrete event in time that:
    • removes biomass
    • alters populations & communities
    • alters structure of ecosystems
    • changes resources availability
    • changes physical environment


  • Natural vs Human caused disturbance
    • lightning, herbivory, flood
    • mining, clear-cutting


  • Maintains communities in non-equilibrium
    • ecosystems rarely reach steady-state

Severity of disturbance: intensity and size


Severity of natural disturbances: frequency


Primary succession




  • After severe disturbances
    • remove or bury biomass
    • no soil or seeds remain


  • Glacial retreat, mud-flows, floodplains, lava flows


  • Colonization:
    • species that handle stresses
    • low nutrients / minimal soil

Primary succession: Facilitators



  • Initial Species composition:
    • nitrogen fixers (bacteria, plants)
    • Facilitation


  • Colonizers must disperse to site
    • wind dispersed seeds/spores
    • algae & lichens
    • small seeded plants
    • distance matters


  • Is primary succession fast or slow?

Secondary succession

Secondary succession: pioneer species





  • Potential growth rate:


  • Tolerance:


  • Seed size & Number:


  • Longevity:

Late succession: inhibition & tolerance

Wildlife and succession

The Boreal: forests shaped by fire

Boreal forest succesion: Aspen - Spruce - Cedar

The future of boreal forests

Study aid: Glacier Bay Alaska succession